The invention relates generally to gamma-ray spectrometers.
Large volume plastic gamma-ray spectrometers constructed using, for example, polyvinyltoluene or polystyrene scintillation material, are being accepted as valid detectors for screening large cargo containers, trucks and trains at ports and other border crossing points. GB 2437979 describes how such detectors might be designed in order to maximise the efficiency with which the scintillation light from a gamma-ray interaction can be collected and demonstrates how the variance in the light-collection efficiency can be minimised. GB 2418015 shows how the most probable spectrum of the incident gamma-rays could be derived from the recoded energy-loss spectrum. The quality of that incident spectrum is such that in many cases it is possible to identify the nature of the radioactivity detected in the cargo. In the radiation detection industry, one goal is to develop a polyvinyl toluene (PVT) can be doped with anthracene to produce a plastic scintillator. When subjected to ionizing radiation (both particle PVT-based system able to identify naturally occurring radioactive materials (NORM) during primary screening to avoid sending the vehicle to a more costly secondary screening process in order to determine whether or not the cargo is safe. The current threshold for the detection of radioactivity in cargo is such that the number of counts detected in a single 20 liter PVT detector is, for example, of the order of 2000 counts per 5 second transit. The sparseness of this data means that it is typical to sum the energy-loss spectrum from four or more large volume PVT detectors.
For this summing process to be useful and not blur the incident gamma-ray spectrum, the energy-scales on each spectrum should be accurately co-aligned. Furthermore, since each of the large detectors is viewed by a number of photomultiplier tubes (e.g., 4 to 8 photomultiplier tubes), in order to maximise the light collection-efficiency of the detector, the gain of each of the photomultiplier tubes is typically normalised so that the overall energy-response of the individual spectrometers is not only uniform across the entire detector surface but is also compensated as the environmental temperature changes or the system ages.
The problem of calibrating the energy-loss spectra generated by a PVT detector is difficult to solve accurately. Often this task is made more difficult by the poor optical design of the large-volume detectors. This means that the quality of the spectra are often so poor that the detailed features used in some calibration methods are difficult to implement. Papers by Kudomi [1] and Siciliano et al. [2] describe how to use features in the energy-loss spectrum of a known monochromatic source of gamma-rays to calibrate the spectrum. A different approach to the calibration of PVT detectors involves the use of a 207Bi source [3]. This generates three monochromatic electrons through an internal-conversion process. More recently, G Pausch et al. [4] have described a stabilisation technique that employs a radioactive calibration source and a secondary scintillation counter to measure the energy of the photon that has been scattered in the PVT.
Therefore, there is a need for an improved technique for more accurately calibrating large scale detectors.